Engine control method and system

ABSTRACT

Methods and systems are provided for heating a catalyst during engine cold-start conditions. One example embodiment uses positive valve overlap to drive a boosted blow-through airflow through the cylinders of an engine. Fuel is injected with the blow-through airflow during the valve overlap, and also injected into engine cylinders outside the valve overlap. The catalyst is heated by the resulting exothermic reaction of the blow-through airflow with the combustion products and the injected fuel in the exhaust manifold.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/075,019, filed Mar. 29, 2011, the entire contents of which areincorporated herein by reference for all purposes.

FIELD

The present description relates generally to a method and system foroperating a combustion engine.

BACKGROUND/SUMMARY

Engine out cold-start emissions generated before light-off of an exhaustsystem catalytic converter may contribute a large percentage of thetotal exhaust emissions. To expedite the attainment of the catalystlight-off temperature, engine systems may inject air into the exhaustmanifold to combust unburned fuel remaining in the exhaust.Additionally, or optionally, the injection of air may be supplementedwith additional fuel to substantially increase the exhaust temperatureand thereby decrease the light-off time.

One example of such an engine system is provided by Uhrich et al. inU.S. 2010/0263639. Herein, during an engine cold-start, an engine isoperated with positive valve overlap while a turbocharger compressor isdriven at least partially by a motor. In this way, a blow-through airflow is generated into the engine exhaust manifold through the cylindersof the engine. Fuel is injected with the blow-through air. Theblow-through air exothermically reacts with the fuel in the exhaust andheats up the exhaust catalyst.

However, the inventors herein have recognized potential issues with sucha system. As one example, the approach relies on a single fuel injectionto heat the engine catalyst as well as attain the desired exhaustair-to-fuel ratio. Since the amount of heat directed to the catalyst isthen adjusted with the blow-through air flow, the system may be heatlimited. As another example, the approach uses a rich cylinder fuelinjection in conjunction with the blow-through airflow to attain thedesired air-to-fuel ratio in the exhaust mixture. However, during someengine cold-start conditions, a lean cylinder fuel injection may bedesired (for example, to reduce exhaust NOx emissions). As such, theapproach of Uhrich is incapable of heating the catalyst and providing astoichiometric exhaust air-to-fuel ratio with a lean injection.

Thus, in one example, some of the above issues may be at least partlyaddressed by a method of operating a boosted engine. One exampleembodiment comprises, during an engine cold start, operating the enginewith positive intake to exhaust valve overlap to drive a boostedblow-through airflow into an engine exhaust through engine cylinders.The method further comprises, injecting a first amount of fuel duringthe valve overlap, injecting a second amount of fuel outside of thevalve overlap, and exothermically reacting the blow-through airflow withfuel in the exhaust.

In one example, a vehicle engine may include a turbocharger coupledbetween the engine intake and the engine exhaust. During an engine coldstart, for example before a catalyst light-off temperature is attained,an engine may be operated with positive intake to exhaust valve overlap,while the turbocharger compressor is operated to drive a boostedblow-through airflow through the engine cylinders, into the exhaustmanifold. During the valve overlap, a first amount of fuel may beinjected into a cylinder, along with the blow-through airflow. A secondamount of fuel may be injected and combusted in the same cylinderoutside of the valve overlap. For example, the second amount of fuel maybe injected after the valve overlap, but while still in the intakestroke of the same combustion cycle. Alternatively, the second amount offuel may be injected outside of (e.g., before) the valve overlap, butwhile still in the intake stroke of the immediately preceding combustioncycle. In still other examples, the first and second amounts may beinjected into different cylinders, the cylinders selected based on theirfiring order. For example, the selection of cylinders may allow theblow-through mixture and the cylinder combustion mixture to be generatedat substantially the same time in the different cylinders and then mixedin the engine exhaust.

As such, the total amount of fuel injected (i.e., first and secondinjection amounts combined) may be adjusted to provide a final desiredexhaust gas mixture air-to-fuel ratio (e.g., around stoichiometry). Asplit ratio of the first injection amount relative to the secondinjection amount in the total amount of fuel injected may be adjustedbased on engine operating conditions, including an exhaust catalysttemperature, to provide a desired heat of oxidation. For example, whenthe exhaust catalyst is at a lower temperature, the first injectionamount may be increased while the second injection amount iscorrespondingly decreased. The resultant rich blow-through air-fuelmixture can be mixed with the lean cylinder combustion mixture togenerate a stoichiometric exhaust gas mixture wherein the richblow-through air-fuel mixture increases the heat delivered to theexhaust catalyst while the lean cylinder combustion reduces cold-startexhaust NOx emission. In an alternate example, when the exhaust catalystis at a higher temperature (but still below the light-off temperature),the first injection amount may be decreased, while the second injectionamount is correspondingly increased. The resultant lean blow-throughair-fuel mixture can be mixed with the rich cylinder combustion mixtureto also generate a stoichiometric exhaust gas mixture wherein the leanblow-through air-fuel mixture decreases the heat delivered to theexhaust catalyst while the rich cylinder combustion is used to maintainengine torque and exhaust air-fuel ratio.

In this way, by injecting some fuel while boosted air is directed thoughthe cylinders, fuel may be mixed thoroughly with blow-through air beforereaching the catalyst. By combusting some fuel in an engine cylinderduring a subsequent intake stroke, and mixing the cylinder combustedexhaust gas with the blow-through air-fuel mixture in the exhaustmanifold, the resultant exhaust gas mixture can be used to expediteattainment of catalyst light-off conditions. Specifically, an exothermicreaction of the blow-through air-fuel mixture with the products of thecylinder combustion (including remaining unburned fuel, and burned fuelproducts such as short chain hydrocarbons (HCs) and carbon-monoxide(CO)) may be promoted to raise the temperature at the exhaust catalyst.By varying the relative amount of fuel in the two injections, the amountof oxidation heat directed to the catalyst can be varied whilemaintaining the exhaust mixture at stoichiometry. By rapidly increasingthe catalyst temperature, the catalyst light-off time may be decreasedand the quality of emissions may be improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of a vehicle system including anengine and an associated exhaust after-treatment system.

FIG. 2 shows a partial engine view.

FIG. 3 shows a map depicting engine positive intake to exhaust valveoverlap.

FIG. 4 shows a schematic depiction of an injection determination routine

FIGS. 5-6 show high level flow charts illustrating routines that may beimplemented for expediting attainment of a catalyst light-offtemperature using a blow-through air-fuel mixture and a cylindercombustion mixture.

FIG. 7 shows a map, depicting engine cycles in different cylinders ofthe engine, that may be used to select cylinders for generating theblow-through air-fuel mixture and the cylinder combustion mixture.

FIG. 8 shows example blow-through air-fuel mixtures and cylindercombustion mixtures that may be used to expedite catalyst heating,according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for reducingthe amount of time needed for a catalyst light-off temperature to beattained in an exhaust after-treatment system coupled to a vehicleengine, as depicted in FIGS. 1-2. By supplying a boosted airchargethrough the engine cylinders, during positive valve overlap (as depictedin FIG. 3), and injecting some fuel with the boosted blow-through air,oxidation heat may be directed towards the exhaust catalyst. Further, byinjecting and combusting additional fuel in an engine cylinder, andcombining the products of the cylinder combustion with the blow-throughmixture at the exhaust manifold, an exothermic reaction may be providedat the exhaust catalyst to substantially increase the catalysttemperature. An engine controller may be configured to perform controlroutines, such as those depicted in FIGS. 5-6, during an engine coldstart, to generate fresh blow-through air flow through the cylinders bydriving an engine boosting device (such as a turbocharger). Thecontroller may further adjust an amount of fuel injected with theblow-through air during the positive valve overlap, and an amount offuel injected into the cylinder outside of the valve overlap based on adesired heat of oxidation. Example maps, such as those shown in FIGS. 4and 7, may be used to determine when to inject the fuel, as well aswhich cylinder to inject the fuel in. Example blow-through air-fuelmixtures and cylinder combustion mixtures that can be used to increasethe exhaust catalyst temperature are shown in FIG. 7. By increasing theexhaust temperature, and expediting attainment of a catalyst light-offtemperature, the quality of vehicle cold-start emissions may besignificantly improved.

FIG. 1 shows a schematic depiction of a vehicle system 6. The vehiclesystem 6 includes an engine system 8 coupled to an exhaustafter-treatment system 22. The engine system 8 may include a boostedengine 10 having a plurality of cylinders 30. Engine 10 includes anengine intake 23 and an engine exhaust 25. Engine intake 23 includes athrottle 62 fluidly coupled to the engine intake manifold 44 via anintake passage 42. The engine exhaust 25 includes an exhaust manifold 48eventually leading to an exhaust passage 35 that routes exhaust gas tothe atmosphere. Throttle 62 may be located in intake passage 42downstream of a boosting device, such as turbocharger 50, or asupercharger. Turbocharger 50 may include a compressor 52, arrangedbetween intake passage 42 and intake manifold 44. Compressor 52 may bepowered by exhaust turbine 54, arranged between exhaust manifold 48 andexhaust passage 35. Compressor 52 may be coupled to exhaust turbine 54via shaft 56. As such, once the engine has run for a sufficient amountof time (for example, a threshold time, a threshold number of combustioncycles, or an amount of time to bring the exhaust gas to a thresholdtemperature), the exhaust gas generated in the exhaust manifold maystart to drive exhaust turbine 54.

Engine exhaust 25 may be coupled to exhaust after-treatment system 22along exhaust passage 35. Exhaust after-treatment system 22 may includeone or more emission control devices 70, which may be mounted in aclose-coupled position in the exhaust passage 35. One or more emissioncontrol devices may include a three-way catalyst, lean NOx filter, SCRcatalyst, etc. The catalysts may enable toxic combustion by-productsgenerated in the exhaust, such as NOx species, unburned hydrocarbons,carbon monoxide, etc., to be catalytically converted to less-toxicproducts before expulsion to the atmosphere. However, the catalyticefficiency of the catalyst may be largely affected by the temperature ofthe exhaust gas. For example, the reduction of NOx species may requirehigher temperatures than the oxidation of carbon monoxide. Unwanted sidereactions may also occur at lower temperatures, such as the productionof ammonia and N₂O species, which may adversely affect the efficiency ofexhaust treatment, and degrade the quality of exhaust emissions. Thus,catalytic treatment of exhaust gas may be delayed until the catalyst(s)have attained a light-off temperature. Additionally, to improve theefficiency of exhaust gas after-treatment, it may be desirable toexpedite the attainment of the catalyst light-off temperature.

As further elaborated herein with reference to FIGS. 5-6, an enginecontroller may be configured to generate a blow-through air-fuel mixturein the exhaust manifold, during an engine cold start, to reduce thelight-off time. The blow-through air-fuel mixture may be generated bydriving an amount of boosted air (herein also referred to as ablow-through airflow), and an associated fuel injection, through theengine cylinders into the exhaust manifold, during a positive intake toexhaust valve overlap period (as shown in FIG. 3). This may besupplemented (e.g. followed or preceded) by the generation of a cylindercombustion mixture by performing a cylinder fuel injection and cylindercombustion, outside of the valve overlap period, but during an intakestroke (e.g. of an intake stroke preceding or succeeding that valveoverlap period). The blow-through mixture may be mixed, andexothermically reacted, with the cylinder combustion mixture in theexhaust manifold to generate a heated exhaust gas mixture that can helpbring the catalytic converter quickly up to an operating temperature.

Exhaust after-treatment system 22 may also include hydrocarbon retainingdevices, particulate matter retaining devices, and other suitableexhaust after-treatment devices (not shown). It will be appreciated thatother components may be included in the engine such as a variety ofvalves and sensors, as further elaborated in the example engine of FIG.2.

The vehicle system 6 may further include control system 14. Controlsystem 14 is shown receiving information from a plurality of sensors 16(various examples of which are described herein) and sending controlsignals to a plurality of actuators 81 (various examples of which aredescribed herein). As one example, sensors 16 may include exhaust gassensor 126 (located in exhaust manifold 48), temperature sensor 128, andpressure sensor 129 (located downstream of emission control device 70).Other sensors such as pressure sensors, temperature sensors, air-to-fuelratio sensors, oxygen sensors, and composition sensors may be coupled tovarious locations in the vehicle system 6, as discussed in more detailherein. As another example, the actuators may include fuel injectors(not shown), a variety of valves, and throttle 62. The control system 14may include a controller 12. The controller may receive input data fromthe various sensors, process the input data, and trigger the actuatorsin response to the processed input data, based on instruction or codeprogrammed therein, corresponding to one or more routines. An examplecontrol routine is described herein with reference to FIGS. 5-6.

FIG. 2 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may be controlled at leastpartially by a control system including controller 12 and by input froma vehicle operator 130 via an input device 132. In this example, inputdevice 132 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Cylinder (i.e.combustion chamber) 30 of engine 10 may include combustion chamber walls136 with piston 138 positioned therein. Piston 138 may be coupled tocrankshaft 140 so that reciprocating motion of the piston is translatedinto rotational motion of the crankshaft. Crankshaft 140 may be coupledto at least one drive wheel of the passenger vehicle via a transmissionsystem. Further, a starter motor may be coupled to crankshaft 140 via aflywheel to enable a starting operation of engine 10.

Cylinder 30 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 30. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 2 shows engine 10configured with a turbocharger including a compressor 52 arrangedbetween intake passages 142 and 144, and an exhaust turbine 54 arrangedalong exhaust passage 148. Compressor 52 may be powered by exhaustturbine 54 via a shaft 56. A throttle 62 including a throttle plate 164may be provided along an intake passage of the engine for varying theflow rate and/or pressure of intake air provided to the enginecylinders. For example, throttle 62 may be disposed downstream ofcompressor 52 as shown in FIG. 2, or may be alternatively providedupstream of compressor 52.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 30. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 70.Sensor 128 may be any suitable sensor for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor.Emission control device 70 may be a three way catalyst (TWC), NOx trap,various other emission control devices, or combinations thereof.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 30 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 30. In some embodiments, eachcylinder of engine 10, including cylinder 30, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 via actuator 152.Similarly, exhaust valve 156 may be controlled by controller 12 viaactuator 154. During some conditions, controller 12 may vary the signalsprovided to actuators 152 and 154 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve150 and exhaust valve 156 may be determined by respective valve positionsensors (not shown). The valve actuators may be of the electric valveactuation type or cam actuation type, or a combination thereof. Theintake and exhaust valve timing may be controlled concurrently or any ofa possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. Each cam actuation system may include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT) and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.For example, cylinder 30 may alternatively include an intake valvecontrolled via electric valve actuation, and an exhaust valve controlledvia cam actuation including CPS and/or VCT. In other embodiments, theintake and exhaust valves may be controlled by a common valve actuatoror actuation system, or a variable valve timing actuator or actuationsystem. The engine may further include a cam position sensor whose datamay be merged with the crankshaft position sensor to determine an engineposition and cam timing.

Cylinder 30 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 30 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 30 is shown including fuel injector 166 coupleddirectly to cylinder 30. Herein, fuel injector 166 is configured as adirect fuel injector for direct injecting fuel into the engine cylinder.Fuel injector 166 may inject fuel directly therein in proportion to thepulse width of signal FPW received from controller 12 via electronicdriver 168. In this manner, fuel injector 166 provides what is known asdirect injection (hereafter referred to as “DI”) of fuel into combustioncylinder 30. While FIG. 2 shows injector 166 as a side injector, it mayalso be located overhead of the piston, such as near the position ofspark plug 192. Alternatively, the injector may be located overhead andnear the intake valve. Fuel may be delivered to fuel injector 166 fromhigh pressure fuel system 172 including a fuel tank, fuel pumps, and afuel rail. Alternatively, fuel may be delivered by a single stage fuelpump at lower pressure. Further, while not shown, the fuel tank may havea pressure transducer providing a signal to controller 12.

It will be appreciated that in an alternate embodiment, injector 166 maybe a port injector providing fuel into the intake port upstream ofcylinder 30. It will also be appreciated that cylinder 30 may receivefuel from a plurality of injectors, such as a plurality of portinjectors, a plurality of direct injectors, or a combination thereof.

Controller 12 is shown in FIG. 2 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type,such as a crankshaft position sensor) coupled to crankshaft 140;throttle position (TP) from a throttle position sensor (not shown); andabsolute manifold pressure signal (MAP) from sensor 124. Engine speedsignal, RPM, may be generated by controller 12 from signal PIP (or thecrankshaft position sensor). Manifold pressure signal MAP from amanifold pressure sensor may be used to provide an indication of vacuum,or pressure, in the intake manifold. Storage medium read-only memory 110can be programmed with computer readable data representing instructionsexecutable by processor 106 for performing the methods described belowas well as other variants that are anticipated but not specificallylisted.

As described above, FIG. 2 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

FIG. 3 shows a map 300 of valve timing and piston position, with respectto an engine position, for a given engine cylinder. During an enginecold-start, an engine controller may be configured to operate an engineboosting device, such as a turbocharger, to drive a boosted blow-throughair flow into the exhaust manifold. The blow-through air flow may bedriven through the engine cylinders while operating the engine withpositive intake to exhaust valve overlap. Additionally, a first amountof fuel may be injected with the blow-through air flow during thepositive overlap period. The injected fuel may be mixed thoroughly withthe blow-through air to generate a blow-through air-fuel mixture that isexothermically reacted in the exhaust manifold to increase an exhaustcatalyst temperature. The engine controller may use a map, such as map300, to identify the positive valve overlap period.

Map 300 illustrates an engine position along the x-axis in crank angledegrees (CAD). Curve 308 depicts piston positions (along the y-axis),with reference to their location from top dead center (TDC) and/orbottom dead center (BDC), and further with reference to their locationwithin the four strokes (intake, compression, power and exhaust) of anengine cycle. As indicated by sinusoidal curve 308, a piston graduallymoves downward from TDC, bottoming out at BDC by the end of the powerstroke. The piston then returns to the top, at TDC, by the end of theexhaust stroke. The piston then again moves back down, towards BDC,during the intake stroke, returning to its original top position at TDCby the end of the compression stroke.

Curves 302 and 304 depict valve timings for an exhaust valve (dashedcurve 302) and an intake valve (solid curve 304) during a normal engineoperation. As illustrated, an exhaust valve may be opened just as thepiston bottoms out at the end of the power stroke. The exhaust valve maythen close as the piston completes the exhaust stroke, remaining open atleast until a subsequent intake stroke has commenced. In the same way,an intake valve may be opened at or before the start of an intakestroke, and may remain open at least until a subsequent compressionstroke has commenced.

As a result of the timing differences between exhaust valve closing andintake valve opening, for a short duration, before the end of theexhaust stroke and after the commencement of the intake stroke, bothintake and exhaust valves may be open. This period, during which bothvalves may be open, is referred to as a positive intake to exhaust valveoverlap 306 (or simply, positive valve overlap), represented by ahatched region at the intersection of curves 302 and 304. In oneexample, the positive intake to exhaust valve overlap 306 may be adefault cam position of the engine present during an engine cold start.

In one example, an engine controller may inject a first amount of fuelduring the positive valve overlap period (that is, when in region 306 ofthe map) to generate a blow-through mixture while injecting a secondamount of fuel in the same combustion cycle after the valve overlapperiod, but before the intake stroke ends (that is, in region 305, whichdoes not overlap with region 306), to generate a combustion mixture. Theblow-through mixture may then be reacted with the cylinder combustionmixture in the exhaust manifold to heat an exhaust catalyst. Aselaborated in the map of FIG. 7, in alternate examples, the controllermay inject the first amount of fuel during the positive overlap periodin one engine cylinder, while injecting the second amount of fuel in thesame cylinder, or an alternate cylinder, outside the valve overlapperiod, but in the intake stroke of a different combustion cycle (e.g.,a preceding or succeeding combustion cycle).

Now turning to FIG. 4, a schematic depiction 400 of an example methodfor adjusting an amount of fuel injected with the blow-through air flowin the blow-through mixture and an amount of fuel in the cylindercombustion mixture, to heat an exhaust catalyst, is shown.

A desired exhaust gas mixture air-to-fuel ratio (AFR) 401 and afeed-forward total air amount 402 may be input into a first controllerK1 to determine a total fuel injection amount 403. As such, the desiredexhaust gas mixture air-to-fuel ratio 401 and the total fuel injectionamount 403 may be determined based on engine operating conditions andsettings. For example, it may be desired to maintain the exhaust gasmixture air-to-fuel ratio substantially at stoichiometry. Likewise,based on engine valve settings, cam settings, boost settings, etc., afeed-forward total air amount 402 may be determined. A second controllerK2 may be configured to determine a split ratio of injections 406 basedon the feed-forward total air amount 402 and the desired exhaust gasmixture air-to-fuel ratio 401. That is, second controller K2 maydetermine how much of the total fuel injection amount is injected into acylinder as a first injection amount with the blow-through air-flow andhow much is injected as a second injection amount in a cylinder (e.g.,same or different cylinder) for combustion with the cylinder combustionair-flow.

Second controller K2 may determine the split ratio 405 based on inputfrom switch 415. Switch 415, in turn, receives inputs regarding adesired cylinder combustion air-to-fuel ratio 420 and a desired heat ofoxidation 412. The desired heat of oxidation 412 is based on deviationof an actual exhaust catalyst temperature 411 from a desired exhaustcatalyst temperature 410. Based on engine operating conditions, and acomparison of the desired heat of oxidation relative to the desired heatof cylinder combustion, a position of switch 415 may be adjusted, and aninput provided to second controller K2 accordingly changed.

As such, for improved exhaust emissions, it may be desired to maintainthe exhaust catalyst at or above a light-off temperature. Thus, forexample, during engine cold-start conditions, when there is a largerdifference between the desired exhaust catalyst temperature and theactual exhaust catalyst temperature, switch 415 may be moved to aposition that gives more weight to the heat of oxidation relative to thecylinder combustion. In comparison, during engine running, when theexhaust catalyst is already warmed up, switch 415 may be moved to aposition that gives more weight to cylinder combustion and maintenanceof a desired cylinder combustion air-to-fuel ratio relative to thedesired heat of oxidation.

Based on the position of switch 415 and the total injection amount 403,the split ratio 405 of injections may be calculated. This includesdetermining a first fuel injection amount 406 and a second fuelinjection amount 408. The first fuel injection amount 406 corresponds tofuel that is injected into an engine cylinder with a boostedblow-through airflow during a positive overlap period while the secondfuel injection amount 408 corresponds to fuel that is injected into anengine cylinder outside the valve overlap period, but within an intakestroke (in the same combustion cycle, or a different combustion cycle).Thus, when a higher heat of oxidation is desired, the split ratio may beadjusted such that more fuel is injected in the first injection (whilecorrespondingly less fuel is injected in the second injection), therebyincreasing the amount of heat directed to the catalyst. In comparison,when maintenance of a desired cylinder combustion air-to-fuel ratio isdesired, the split ratio may be adjusted such that more fuel is injectedin the second injection (while correspondingly less fuel is injected inthe first injection) to decrease the amount of heat directed to thecatalyst and increase cylinder combustion. In this way, by varying therelative amount of fuel in the two injections, the amount of oxidationheat directed to the catalyst can be varied while maintaining theexhaust mixture at a desired air-to-fuel ratio.

Now turning to FIG. 5, an example routine 500 is shown for performing asupplementary blow-through air and fuel injection operation during anengine cold start, while operating the engine with positive intake toexhaust overlap, in the vehicle system of FIG. 1. The routine enablesthe compressor of an engine intake boosting device to be operated togenerate blow-through air flow in the exhaust while some fuel isinjected and mixed with the blow-through air-flow to generate ablow-through mixture in the exhaust manifold. Upon mixing theblow-through mixture with combusted exhaust gas from a subsequentcylinder combustion event, an exhaust gas mixture may be generated inthe exhaust manifold. In doing so, exothermic events in the exhaustmanifold may be promoted and an exhaust temperature may be rapidlyincreased, thereby reducing a catalyst light-off time.

At 502, an engine cold start condition may be confirmed. The engine coldstart condition may include at least one of an exhaust catalysttemperature being below a threshold temperature (such as a light-offtemperature), and the engine being in an engine-off condition forgreater than a threshold duration. If an engine cold start condition isnot present, the routine may end. At 504, it may be determined whetherthe exhaust temperature has reached a threshold temperature. As such,the threshold temperature may correspond to an exhaust temperature abovewhich exhaust gas may start to drive the turbocharger turbine. In otherwords, the turbine may not be operated (without assist), and no boostmay be generated, for a number of combustion cycles since the enginecold-start, until the exhaust temperature rises to the thresholdtemperature. Upon confirmation, at 506, intake and/or exhaust valvetimings may be adjusted to operate the engine with a positive intake toexhaust valve overlap. In one example, a positive valve overlap may bethe default cam position such that the positive valve overlap is presentat the time of engine cold start. An engine controller may be configuredto use a map, such the map of FIG. 3, to identify cam timingscorresponding to the desired positive intake to exhaust valve overlapperiod. Also at 506, turbocharger settings may be adjusted (e.g., aturbocharger compressor may be operated) to generate positive boostpressure.

At 508, engine operating conditions may be estimated, and/or measured.As such, these may include, but not be limited to, engine temperature,engine coolant temperature, exhaust temperature, catalyst temperature,engine speed, manifold pressure, barometric pressure, etc. In oneexample, the catalyst temperature may be inferred from the exhausttemperature. In another example, the catalyst temperature and/or theexhaust temperature may be further compared to a threshold temperature,such as a catalyst light-off temperature, and a temperature differencemay be determined.

At 510, based on the estimated engine operating conditions, including acomparison of the desired heat of oxidation relative to the desiredcylinder combustion air-to-fuel ratio (or cylinder heat), as well as adesired exhaust air-to-fuel ratio, the first and second injectionamounts may be determined. As elaborated in FIGS. 4 and 6, this includesadjusting a split ratio of the first injection amount relative to thesecond injection amount based at least on an exhaust catalysttemperature. For example, the first injection amount may be increasedrelative to the second injection amount as a difference between theexhaust catalyst temperature and a threshold temperature (e.g.,light-off temperature) increases, while maintaining a total injectionamount constant. However, the sum of the first and second injectionamounts may be adjusted to maintain an overall air-to-fuel ratio in theexhaust at a desired air-to-fuel ratio (e.g., substantially atstoichiometry).

At 512, the first amount of fuel may be injected during the positivevalve overlap period along with the boosted blow-through airflow (thatis, an exhaust stroke injection). In this way, a boosted blow-throughmixture may be driven into the engine exhaust through the enginecylinders. At 514, it may be confirmed whether the positive valveoverlap period is over. If yes, then at 516, the controller may injectthe second amount of fuel in the same combustion cycle as the firstinjection, after the valve overlap but before the intake valve closes(that is, within the intake stroke). In one example, the enginecontroller may use the map of FIG. 3 to identify cam timings where thepositive valve overlap period is over but the intake valve has notclosed. The second injection amount may be combusted in the cylinder togenerate a cylinder combustion mixture.

As a result of the injections, the blow-through mixture mayexothermically react with the cylinder combustion mixture in the exhaustmanifold, near the exhaust catalyst, thereby heating the catalyst. At518, it may be confirmed whether the temperature of the exhaust catalystis at or above a threshold temperature, such as a light-off temperature(T_(light-off)). If the light-off temperature has not been attained,that is, the catalyst has not been sufficiently heated, then at 522, theroutine may return to 510 to continue adjusting a fuel distributionbetween the first and the second injection amounts to provide a desiredheat of oxidation. If the light-off temperature has been attained, thatis, the catalyst has been sufficiently heated, then at 520, the firstblow-through operation may be discontinued while the second fuelinjection is adjusted based on the desired cylinder combustion andexhaust air-to-fuel ratio. In one example, discontinuing the firstblow-through operation may include discontinuing only the first fuelinjection while adjusting the turbocharger settings based on the engineoperating conditions and desired torque.

Now turning to FIG. 6, an example routine 600 is shown for adjusting adistribution of the first and second injection amounts based on engineoperating conditions. The routine of FIG. 6 may be performed as part ofthe routine of FIG. 5, specifically at 510.

At 602, a desired exhaust gas mixture air-to-fuel ratio may bedetermined based on engine operating conditions. As such, this maycorrespond to the air-to-fuel ratio of a final exhaust gas mixturegenerated following release of cylinder combustion products into theexhaust manifold and exothermic reaction of the cylinder combustionproducts with a blow-through air-fuel mixture. At 604, a feed-forwardtotal air amount may be received based on intake and/or exhaust valvesettings and turbocharger settings. At 606, a total injection amount maybe determined based on the feed-forward total air amount and the desiredexhaust gas mixture air-to-fuel ratio.

At 608, a desired heat of oxidation may be determined based on engineoperating conditions, including at least the catalyst temperature. Forexample, the actual catalyst temperature may be compared to a desiredcatalyst temperature (such as a catalyst light-off temperature) and thedesired heat of oxidation may be determined based on the differencebetween the two. At 610, desired cylinder combustion conditionsincluding a desired cylinder combustion air-to-fuel ratio, desiredcylinder heat of combustion, desired cylinder temperature, etc., may bedetermined based on engine operating conditions.

At 612, a split ratio (or distribution) of a first injection amount,used to generate the blow-through air-fuel mixture, relative to a secondinjection amount, used to generate the cylinder combustion air-fuelmixture, is determined. At 614, based on the split ratio, the first andsecond injection amounts may be determined.

As such, the distribution of the total injection amount between thefirst and the second injection amounts determines the amount of heatthat is directed to the exhaust catalyst. Since the first amount of fuelis injected during a positive valve overlap period, along with a boostedblow-through air flow (generated by operating a turbocharger during thepositive valve overlap period), the fraction of fuel injected as thefirst injection amount largely accounts for heating the exhaustmanifold. In comparison, since the second amount of fuel is injectedinto the cylinder after the positive valve overlap period, but whilestill in the intake stroke, the fraction of fuel injected as the secondinjection amount largely accounts for the cylinder combustion.Therefore, by increasing the fraction of fuel injected in the firstinjection amount, the heat of oxidation directed to the exhaust catalystcan be increased.

In one example, the split ratio of the first injection amount relativeto the second injection amount may be adjusted based on an exhaustcatalyst temperature, for example, by increasing the ratio of the firstinjection amount when the exhaust catalyst temperature is below athreshold temperature (e.g., light-off temperature). Irrespective of thedistribution of fuel between the first and second injection amounts, asum of the first and second injection amounts may be maintained constantso as to maintain an air-to-fuel ratio of the resulting exhaust gasmixture at a desired air-to-fuel ratio. For example, the sum of theinjection amounts may be adjusted to maintain the exhaust gas mixturesubstantially at or around stoichiometry.

At 616, it may be determined whether any further air-to-fuel ratio (AFR)adjustments are needed. In one example, the engine may include anair-to-fuel ratio sensor in the engine exhaust, such as an exhaust gasoxygen (EGO) sensor. Feedback from the air-fuel ratio sensor may be usedto adjust the overall air-to-fuel ratio in the exhaust gas by adjustingthe first and/or second injection amounts. In this way, the feedback maybe used to perform further adjustments to the blow-through air-fuelmixture and/or the cylinder combustion air-fuel mixture, to therebyadjust the resulting engine exhaust mixture air-to-fuel ratio. In oneexample, the adjustments made based on feedback from the air-fuel ratiosensor may cause the final exhaust mixture air-to-fuel ratio tooscillate around stoichiometry.

If no air-to-fuel ratio (AFR) adjustments are needed, the routine mayend. If air-to-fuel ratio adjustments are needed, then at 618, the firstand second injection amounts are further adjusted based on theair-to-fuel ratio feedback from the exhaust gas oxygen sensor, oralternate exhaust gas air-to-fuel ratio sensor. The air-to-fuel ratioadjustments may include, for example, during a first condition,adjusting the first injection amount but not the second injection amountresponsive to the feedback. The first condition may include an enginecold-start condition. As such, during the cold-start condition, it maybe crucial to bring the exhaust catalyst up to a light-off temperature,to reduce exhaust emissions. Thus, by adjusting only the first injectionamount responsive to the air-to-fuel ratio feedback during cold-startconditions, a desired heat of oxidation may be accurately provided toheat the exhaust catalyst.

As another example, during a second condition, the second injectionamount but not the first injection amount may be adjusted responsive tothe feedback. As yet another example, during a third condition, each ofthe first and the second injection amount may be adjusted responsive tothe feedback. In this way, by adjusting the first and/or secondinjection amounts responsive to air-to-fuel ratio feedback from anexhaust gas oxygen sensor, a desired heat of oxidation and a desiredcylinder combustion may be achieved, while maintaining the final exhaustgas mixture at the desired air-to-fuel ratio.

It will be appreciated that the while the routines depicted in FIGS. 5-6illustrate a second amount of fuel being injected in the intake strokeof the same combustion cycle as the first injection amount, this is notmeant to be limiting. In alternate examples, the second injection amountmay be injected in an intake stroke of a different combustion cycle asthe first injection amount. The different combustion cycle may be, forexample, a combustion cycle immediately preceding or immediatelyfollowing the combustion cycle of the first injection amount. Thus,while the depicted example illustrates mixing the blow-through mixtureof a cylinder with the products of a subsequent (e.g., immediatelyensuing) cylinder combustion, in alternate examples, the secondinjection may precede the first injection such that the blow-throughmixture is mixed with the products of a preceding (e.g., immediatelypreceding) cylinder combustion. In this case, the second injection maybe performed before the intake valve closes for an intake strokeimmediately preceding the valve overlap period where the first injectionis performed. The first injection may precede the second injection orvice versa.

It will also be appreciated that while the depicted routine shows thefirst and second injection amounts being injected into the samecylinder, this is also not meant to be limiting. In alternate examples,as elaborated with reference to FIG. 7, the first injection amount maybe injected in a first cylinder while the second injection amount isinjected in a second cylinder, the first and second cylinders selectedbased on their firing order.

FIG. 7 shows an example map of engine cylinder positions for differentcylinders of an engine. In the depicted example, four cylinders(cylinders 1-4) of an in-line engine are shown in the correspondingfiring order.

Based on the position of each cylinder's piston at any time in theengine cycle, a given cylinder may be in an intake stroke (I), acompression stroke (C), a power stroke (P), or an exhaust stroke (E).For any given cylinder, the positive valve overlap period corresponds toa narrow window (represented herein by a hatched bar) towards the end ofthe exhaust stroke and the beginning of the intake stroke of the givencylinder. A controller may inject the first amount of fuel in a firstcylinder during the positive valve overlap period of the first cylinder.The controller may then select a second, different cylinder, forinjecting the second amount of fuel within an intake stroke of thesecond cylinder, but outside of the positive valve overlap period of thesecond cylinder.

The first and second cylinders may be selected based on their firingorder. For example, cylinders may be selected such that the blow-throughmixture and the cylinder combustion mixture are generated atsubstantially the same time. In one example, the cylinder selectionincludes a first cylinder and a second cylinder wherein a positive valveoverlap period of the first cylinder overlaps the exhaust stroke(depicted herein by a bar with diagonal lines) of the second cylinder.Since the cylinder combustion mixture is exhausted in the exhaust strokeof the cylinder, by injecting the first amount of fuel in the valveoverlap period of the first cylinder and the second amount of fuel inthe intake stroke of a second cylinder that has an exhaust stroke thatoverlaps the valve overlap period of the first cylinder, a mixing andexothermic reaction of the blow-through mixture with the cylindercombustion mixture in the exhaust manifold may be improved.

With reference to the map of FIG. 7, as a non-limiting example, theengine controller may inject the first amount in Cylinder 2 during thevalve overlap period, while injecting the second amount in Cylinder 1,during the intake stroke of a preceding combustion cycle. As anotherexample, the engine controller may inject the first amount in Cylinder 3during the valve overlap period, while injecting the second amount inCylinder 4, during the intake stroke of a preceding combustion cycle. Assuch, still other combinations may be possible.

Now turning to FIG. 8, example blow-through air-fuel mixtures andcylinder combustion mixtures are depicted that may be used to expeditecatalyst heating. In particular, example distributions of fuel between afirst injection amount in a blow-through mixture, and a second injectionamount in a cylinder combustion mixture, are shown. In all the depictedexamples, the different distributions are used to vary the amount ofheat delivered to an exhaust catalyst while maintaining a final exhaustgas mixture air-to-fuel ratio substantially at stoichiometry.

For each of the blow-through mixture and the cylinder combustionmixture, a feed-forward air amount may be determined (represented hereinby corresponding hatched bars). The feed-forward air amount for theblow-through mixture as well as the cylinder combustion mixture may bebased on boost settings of the turbocharger, as well as intake andexhaust valve settings (e.g., valve opening, valve closing, duration ofvalve opening, duration of valve overlap, etc.). In the depictedexample, the blow-through mixture may include a larger amount of air ascompared to the cylinder combustion mixture.

Example I depicts a first example of a fuel distribution (representedherein by corresponding solid bars) between the blow-through mixture andthe cylinder combustion mixture. Herein, a first injection amount isadjusted to be proportional to the blow-through air amount so that asubstantially stoichiometric blow-through mixture is generated.Likewise, a second injection amount is adjusted to be proportional tothe cylinder combustion air amount so that a substantiallystoichiometric cylinder combustion mixture is generated. Thus, inexample I, a stoichiometric blow-through air-fuel mixture may be mixedwith a stoichiometric cylinder combustion air-fuel mixture in theexhaust manifold to generate heat at the exhaust catalyst, whilemaintaining the resulting exhaust gas mixture at stoichiometry.

As such, it will be appreciated that herein the fuel amount beingproportional to the air amount does not imply a 1:1 air:fuel ratio.Rather, it implies a ratio required to generate a stoichiometric mixture(e.g., 14.6:1 air:fuel). However, in alternate examples, the air:fuelratio may be different (e.g., a 1:1 ratio) based on the desiredair-to-fuel ratio of each mixture.

Example II depicts another example fuel distribution. Herein, the firstinjection amount is adjusted to be larger in proportion to theblow-through air amount so that a rich blow-through mixture isgenerated. The second injection amount is correspondingly decreased tomaintain the total injection amount constant. Consequently, the secondinjection amount is smaller in proportion to the cylinder combustion airamount such that a lean cylinder combustion mixture is generated. Thus,in example II, a rich blow-through air-fuel mixture may be mixed with alean cylinder combustion air-fuel mixture in the exhaust manifold togenerate more heat (relative to examples I and III) at the exhaustcatalyst while maintaining the resulting exhaust gas mixture atstoichiometry. The fuel distribution of example II may be used during afirst engine cold-start condition wherein the temperature of the exhaustcatalyst is lower than a threshold and below a catalyst light-offtemperature.

Example III depicts yet another example fuel distribution. Herein, thefirst injection amount is adjusted to be smaller in proportion to theblow-through air amount so that a lean blow-through mixture isgenerated. The second injection amount is correspondingly increased tomaintain the total injection amount constant. Consequently, the secondinjection amount is larger in proportion to the cylinder combustion airamount such that a rich cylinder combustion mixture is generated. Thus,in example III, a lean blow-through air-fuel mixture may be mixed with arich cylinder combustion air-fuel mixture in the exhaust manifold togenerate less heat (relative to examples I and II) at the exhaustcatalyst while maintaining the resulting exhaust gas mixture atstoichiometry. The fuel distribution of example III may be used during asecond engine cold-start condition wherein the temperature of theexhaust catalyst is higher than the threshold but still below thelight-off temperature. Alternatively, the fuel distribution of exampleIII may be used when a richer cylinder combustion is desired to limitNOx emissions and/or meet higher torque demands.

Example IV depicts yet another example wherein instead of distributingfuel between a first and second injection, all the fuel is injected inone injection. Specifically, boosted blow-through air, with no addedfuel, is directed to the engine exhaust through the engine cylinders,during the valve overlap period. The boosted blow-through air flow isthen exothermically reacted with a rich cylinder combustion mixture inthe exhaust manifold to generate more heat at the exhaust catalyst whilemaintaining the resulting exhaust gas mixture at stoichiometry. Herein,the cylinder combustion may either precede the boosted blow-through airflow, such that the fresh air of the blow-through air flow isexothermically reacted with the combustion products of a precedingcylinder combustion event, or the cylinder combustion may immediatelyfollow the boosted blow-through air flow, such that the fresh air of theblow-through air flow is exothermically reacted with the combustionproducts of an immediately ensuing cylinder combustion event.

In this way, by injecting some fuel into an engine cylinder along with aboosted blow-through air flow, through the cylinder and into the engineexhaust, a well-mixed blow-through air-fuel mixture may be generated atthe exhaust. By injecting and combusting some fuel in an engine cylinderduring the intake stroke, a cylinder combustion mixture may begenerated. By mixing the blow-through air-fuel mixture with the cylindercombustion products in the exhaust, a combustion reaction may begenerated that increases the heat at an exhaust emission control deviceand expedites attainment of catalyst light-off temperatures. By varyingthe distribution of fuel injected in a cylinder with the blow-throughairflow as compared to fuel combusted in a cylinder, the amount ofoxidation heat directed to the catalyst can be varied while maintainingthe exhaust mixture at a desired air-to-fuel ratio. By rapidlyincreasing the catalyst temperature, the catalyst light-off time may bedecreased and the quality of emissions may be improved.

Note that the example control and estimation routines included hereincan be used with various system configurations. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations, orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated actions,functions, or operations may be repeatedly performed depending on theparticular strategy being used. Further, the described operations,functions, and/or acts may graphically represent code to be programmedinto computer readable storage medium in the control system

Further still, it should be understood that the systems and methodsdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are contemplated. Accordingly, the presentdisclosure includes all novel and non-obvious combinations of thevarious systems and methods disclosed herein, as well as any and allequivalents thereof.

The invention claimed is:
 1. A method of operating an engine,comprising: operating the engine in a first mode if a temperature of anexhaust catalyst is below a threshold and in a second mode if thetemperature of the exhaust catalyst is above the threshold; during afirst engine cold-start with the engine operating in the first mode,mixing a rich blow-through air-fuel mixture with a lean cylindercombustion air-fuel mixture in an exhaust manifold to generate more heatat an exhaust catalyst; and during a second engine cold-start with theengine operating in the second mode, mixing a lean blow-through air-fuelmixture with a rich cylinder combustion air-fuel mixture in the exhaustmanifold to generate less heat at the exhaust catalyst.
 2. The method ofclaim 1, wherein during the first and second engine cold-starts, theblow-through air-fuel mixture is a boosted blow-through air-fuel mixturegenerated by operating a turbocharger during a positive intake toexhaust valve overlap period while injecting a first amount of fuelduring the positive intake to exhaust valve overlap period and thecylinder combustion air-fuel mixture is generated by injecting a secondamount of fuel outside of the positive intake to exhaust valve overlapperiod but before an intake valve closes.
 3. The method of claim 2,wherein the cylinder combustion air-fuel mixture and the boostedblow-through air-fuel mixture are generated in a same cylinder in a samecombustion cycle.
 4. The method of claim 2, wherein during the first andsecond engine cold-starts, an exhaust gas mixture in the exhaustmanifold is maintained substantially at stoichiometry.
 5. The method ofclaim 4, wherein maintaining the exhaust gas mixture in the exhaustmanifold substantially at stoichiometry includes adjusting the firstand/or second injection amount based on feedback received from anexhaust air-to-fuel ratio sensor.
 6. The method of claim 1, furthercomprising: during the first engine cold-start, injecting a first amountof fuel and driving an amount of boosted air through engine cylindersinto the exhaust manifold, during a positive intake to exhaust valveoverlap period, to generate the rich blow-through air-fuel mixture, andinjecting a second amount of fuel after valve overlap but before anintake valve closes to generate the lean cylinder combustion air-fuelmixture; and during the second engine cold-start, injecting a thirdamount of fuel and driving an amount of boosted air through enginecylinders into the exhaust manifold, during a positive intake to exhaustvalve overlap period, to generate the lean blow-through air-fuelmixture, and injecting a fourth amount of fuel after valve overlap butbefore the intake valve closes to generate the rich cylinder combustionair-fuel mixture.
 7. The method of claim 6, further comprising: duringthe first engine cold-start, adjusting a sum of the first and secondinjection amounts to maintain an air-to-fuel ratio of an exhaust gasmixture in the exhaust manifold around stoichiometry while a ratio ofthe first injection amount relative to the second injection amount isadjusted based at least on exhaust catalyst temperature; and during thesecond engine cold-start, adjusting a sum of the third and fourthinjection amounts to maintain an air-to-fuel ratio of an exhaust gasmixture in the exhaust manifold around stoichiometry while a ratio ofthe third injection amount relative to the fourth injection amount isadjusted based at least on exhaust catalyst temperature.
 8. The methodof claim 6 further comprising, during a third engine cold-start with theengine operating in a third mode, injecting a fifth amount of fuel anddriving an amount of boosted air through engine cylinders into theexhaust manifold, during a positive intake to exhaust valve overlapperiod, to generate a stoichiometric blow-through air-fuel mixture, andinjecting a sixth amount of fuel after valve overlap but before theintake valve closes to generate a stoichiometric cylinder combustionair-fuel mixture.
 9. The method of claim 8 further comprising: duringthe first engine cold-start, adjusting the first and second injectionamounts based on air-to-fuel ratio feedback from an exhaust gas oxygensensor; during the second engine cold-start, adjusting the third andfourth injection amounts based on air-to-fuel ratio feedback from theexhaust gas oxygen sensor; and during the third engine cold-start,adjusting the fifth and sixth injection amounts based on air-to-fuelratio feedback from the exhaust gas oxygen sensor.
 10. The method ofclaim 9, wherein adjusting the first and second injection amounts basedon air-to-fuel ratio feedback from the exhaust gas oxygen sensorincludes: during the first engine cold-start, during a first condition,adjusting the first injection amount but not the second injection amountresponsive to the feedback; during a second condition, adjusting thesecond injection amount but not the first injection amount responsive tothe feedback; and during a third condition, adjusting the first and thesecond injection amounts responsive to the feedback; and during thesecond engine cold-start, during a fourth condition, adjusting the thirdinjection amount but not the fourth injection amount responsive to thefeedback; during a fifth condition, adjusting the fourth injectionamount but not the third injection amount responsive to the feedback;and during a sixth condition, adjusting the third and the fourthinjection amounts responsive to the feedback.
 11. The method of claim 1,further comprising: during the first engine cold-start, injecting afirst amount of fuel in a first cylinder and driving a first amount ofboosted air through the first cylinder into an exhaust manifold, duringa positive intake to exhaust valve overlap period of the first cylinder,to generate the rich blow-through air-fuel mixture, and injecting asecond amount of fuel in a second cylinder after a positive intake toexhaust valve overlap period of the second cylinder but before an intakevalve of the second cylinder closes to generate the lean cylindercombustion air-fuel mixture in the second cylinder; and during thesecond engine cold-start, injecting a third amount of fuel in the firstcylinder and driving a second amount of boosted air through the firstcylinder into the exhaust manifold, during a positive intake to exhaustvalve overlap period of the first cylinder, to generate the leanblow-through air-fuel mixture, and injecting a fourth amount of fuel inthe second cylinder after a positive intake to exhaust valve overlapperiod of the second cylinder but before the intake valve of the secondcylinder closes to generate the rich cylinder combustion air-fuelmixture in the second cylinder.
 12. The method of claim 2 furthercomprising, during the first engine cold-start, exothermically reactingthe rich blow-through air-fuel mixture with the lean cylinder combustionair-fuel mixture in the exhaust manifold, and during the second enginecold-start, exothermically reacting the lean blow-through air-fuelmixture with the rich cylinder combustion air-fuel mixture in theexhaust manifold.
 13. An engine method, comprising: injecting a firstamount of fuel and driving an amount of boosted air through enginecylinders into an exhaust manifold, during a positive intake to exhaustvalve overlap period, to generate a blow-through mixture; injecting asecond amount of fuel after valve overlap but before an intake valvecloses to generate a cylinder combustion mixture; and exothermicallyreacting the blow-through mixture with the cylinder combustion mixturein the exhaust manifold.
 14. The method of claim 13, further comprisingadjusting a sum of the first and second injection amounts to maintain anair-to-fuel ratio of an exhaust gas mixture in the exhaust manifoldaround stoichiometry while a ratio of the first injection amountrelative to the second injection amount is adjusted based at least onexhaust catalyst temperature.
 15. The method of claim 14, furthercomprising: when exhaust catalyst temperature is lower than a threshold,adjusting the first injection amount to achieve a rich blow-throughair-fuel mixture, and adjusting the second injection amount to achieve alean cylinder combustion mixture; and when exhaust catalyst temperatureis higher than the threshold, adjusting the first injection amount toachieve a lean blow-through air-fuel mixture, and adjusting the secondinjection amount to achieve a rich cylinder combustion mixture.
 16. Amethod for an engine, comprising: injecting a first amount of fuel in afirst cylinder and driving an amount of boosted air through the firstcylinder into an exhaust manifold, during a positive intake to exhaustvalve overlap period of the first cylinder, to generate a blow-throughmixture; and injecting a second amount of fuel in a second cylinderafter a positive intake to exhaust valve overlap period of the secondcylinder but before an intake valve of the second cylinder closes togenerate a cylinder combustion mixture in the second cylinder; andexothermically reacting the blow-through mixture with the cylindercombustion mixture of the second cylinder in the exhaust manifold. 17.The method of claim 16, wherein the positive intake to exhaust valveoverlap period of the first cylinder overlaps an exhaust stroke of thesecond cylinder.
 18. The method of claim 16, further comprising:adjusting a sum of the first and second injection amounts to maintain anair-to-fuel ratio of an exhaust gas mixture in the exhaust manifoldaround stoichiometry while a ratio of the first injection amountrelative to the second injection amount is adjusted based at least onexhaust catalyst temperature.